The present invention relates to a method for manufacturing an arc tube used as a light source for a metal halide lamp employed as a discharge bulb for a motor vehicle headlamp or the like.
A discharge bulb for a motor vehicle headlamp or the like includes an arc tube composed of a glass tube containing mercury or metal halide as a luminous material, and a rare gas. The discharge bulb is superior to a bulb of the filament type in that the former is free from failure caused by filament burnout, and has the benefit of a large amount of light emitted. For this reason, the discharge bulb has been given a great amount of attention recently.
As shown in FIG. 19, an arc tube 1 is composed of a glass tube 1a pinch sealed at both ends thereof. Metal halide (scandium iodide (ScI.sub.3), sodium iodide (NaI), and the like) as a luminous material, mercury and rare gas (Xe, Ar or the like) is contained in the central portion of the glass tube. A lead wire a, a molybdenum foil b, and an electrode (bar) c are assembled into a single unit (electrode assembly). The electrode assemblies are respectively attached to the pinch sealed portions 1b in a sealed fashion, as shown. A pair of electrodes (bars) c are disposed opposite one another within the glass tube 1a.
To manufacture the arc tube 1, a T-shaped glass tube W as shown in FIG. 20 is first provided, in which a exhaust tube 2 is connected to the glass tube 1a of the arc tube 1. A degassing process to remove impurities from the glass tube 1a is carried out. In the degassing process, the glass tube 1a is degassed by heating the tube without connecting an exhaust device (not shown) through a connection head 10 to the exhaust tube 2. Subsequently, an ion bombardment process is carried out. For this process, inert gas is introduced into the glass tube la through the exhaust tube 2. The electrodes thereof are connected to a glow discharge generating circuit 6. Current is fed to the electrodes to thereby cause a discharge between the electrodes. As a result, impurities adhering to the surface of the electrodes c are gasified and discharged therefrom. Thereafter, metal halide (scandium iodide (ScI.sub.3), sodium iodide (NaI), and the like), mercury, and a rare gas (Xe, Ar or the like) are successively introduced into the glass tube 1a. Then, the exhaust tube 2 is tipped off.
The glow discharging circuit 6 is constructed such that the electrodes of the arc tube are connected to the secondary coil of a booster transformer T1 through a current limiting coil L1. The primary coil of the booster transformer T1 is connected to an AC power source (e.g., 200 V) through a switch SW1.
The ion bombardment process that is carried out before the introduction of metal halide (scandium iodide (ScI.sub.3), sodium iodide (NaI), and the like), mercury, and rare gas, is effective in removing impurities (mainly oxide) adhering to the electrodes and improving the luminous flux retention rate. For this reason, the ion bombardment process is indispensable for the arc tube manufacturing process.
When an impurity (oxygen) is present in the glass tube 1a, ScI.sub.3 is chemically transformed, as indicated by the following equation: EQU 4ScI.sub.3 +4SiO2.sub.2 +3O.sub.2 +6Hg.fwdarw.2Sc.sub.2 Si.sub.2 O.sub.7 +6HgI.sub.2
From this, it may be presumed that the luminous flux is reduced when oxygen is present as an impurity. To control the reduction of the luminous flux, it is desirable to remove the impurities (particularly oxygen) from the inside of the glass tube 1a before the introduction of metal halide (scandium iodide (ScI.sub.3), sodium iodide (NaI), and the like), mercury, and a rare gas.
In the conventional ion bombardment process, current of a relatively low current density (several mA/mm.sup.2 to several tens mA/mm.sup.2) is fed to the electrodes c by the glow discharge generating circuit 6. Accordingly, the discharge caused between the electrodes c is a glow discharge. The glow discharge is not capable of sufficiently removing the impurities from the electrode surfaces and providing a satisfactorily high luminous flux retention rate.
In the conventional arc tube manufacturing method, the ion bombardment process follows the degassing process. Accordingly, there is the possibility that oxide material scattered from the electrode surfaces during the glow discharge in the ion bombardment process will adhere to the wall of the glass tube 1a, and thus oxide material is left on the tube wall after the ion bombardment process.